A classic, central Service-Oriented Computing (SOC) challenge is the service composition problem. It concerns solving a user-defined task by selecting a suitable set of services, possibly found at runtime, determining an invocation order, and handling request and response parameters. The solutions proposed in the past two decades mostly resort to additional formal modeling of the services, leading to extra effort, scalability issues, and overall brittleness. With the rise of Large Language Models (LLMs), it has become feasible to process semi-structured information like state-of-the-practice OpenAPI documentation containing formal parts like endpoints and free-form elements like descriptions. We propose Compositio Prompto to generate service compositions based on those semi-structured documents. Compositio Prompto acts as an encapsulation of the prompt creation and the model invocation such that the user only has to provide the service specifications, the task, and which input and output format they expect, eliminating any manual and laborious annotation or modeling task by relying on already existing documentation. To validate our approach, we implement a fully operational prototype, which operates on a set of OpenAPIs, a plain text task, and an input and output JSON schema as input and returns the generated service composition as executable Python code. We measure the effectiveness of our approach on a parking spot booking case study. Our experiments show that models can solve several tasks, especially those above 70B parameters, but none can fulfill all tasks. Furthermore, compared with manually created sample solutions, the ones generated by LLMs appear to be close approximations.